Demodulator for an amplitude-modulated alternating signal

ABSTRACT

The invention concerns a demodulator of an amplitude-modulated signal (Vdb), characterised in that it comprises a peak detecting cell (DCR) capable of extracting the reference modulating signal (Vpeak 1 ) of the modulated signal (Vdb); a first demodulator (FE) adapted to detect the peak of the reference modulating signal (Vpeak 1 ) to generate a high comparison threshold and locate the start of the modulation, a second demodulator (RE) adapted to detect a trough of the reference modulating signal (Vpeak 1 ) to generate a low comparison threshold and locate the end of the modulation; a logic processing unit capable of supplying the demodulated signal (Vdemod).

FIELD OF THE INVENTION

The present invention relates to a demodulator for an amplitudemodulated alternating signal based on the peak and trough detectionprinciple. The invention also relates to a contactless device comprisingsuch a demodulator.

The demodulator according to the invention has a particularlyadvantageous application in the field of long range radio frequency (RF)communication.

BACKGROUND OF THE INVENTION

Contactless communication portable devices such as contactless IC(integrated circuit) cards (also known as chip cards or smart cards),electronic labels or tags, or badges, operate on the basis of acommunication by an electromagnetic field with a read and/or writeinterrogating device, generically referred to as a reader. Suchcontactless devices generally comprise a microcircuit connected to aparallel LC type resonant circuit. The inductor is an external antenna,while the capacitor is integrated to the microcircuit. The two form whatis commonly known as a “tuned circuit”.

As an example, in some contactless IC card applications, the readertransmits an electromagnetic signal having a carrier frequency of 13.56MHz.

This transmitted signal serves on the one hand to power the contactlesscard, which thus derives by induction the energy required for itsoperation, and on the other hand to set up a communication between thecard and the reader according to an established protocol. Thus, when thecontactless card penetrates into the transmission field of the reader,it communicates with the latter by a modulation operation which consistsin modifying at least one parameter of the carrier.

The contactless device receives, via its tuned circuit, an amplitudemodulated signal from the reader. The contactless device interprets themessage from the reader by a demodulation operation which consists inextracting the modulated signal from the carrier. The frequency of themodulated signal is much smaller than that of the carrier, in generalaround ten kHz.

The quality and reliability of the RF communication are directly linked,among things, to the distance between the reader and the contactlessdevice. The distance, or range, of the RF communication between thereader and the contactless device depends on several parametersincluding the tuning frequency between the resonant circuit of thecontactless device and the transmission frequency of the reader, as wellas the quality of demodulation of the modulated signal.

The quality of demodulation of the modulated signal depends directly onthe distance between the contactless device and the reader, as well asthe speed of displacement of the device in the transmission field of thereader. The greater is the range, and the more the device is stealthy,the more the demodulation shall be subject to error.

FIG. 1 is a block diagram of the input stages of the contactless device.A resonant circuit, centered at the carrier frequency, receives amodulated electromagnetic signal. A rectifier bridge generates a dcvoltage in order to power the contactless device. The output voltage Vdbof the rectifier bridge represents the dc voltage after rectificationand contains both the energy needed for self-powering the contactlessdevice and the information of the modulated signal.

For applications that use 100% amplitude modulation, a diode isolatesthe resonant circuit from the load and thus eliminates all possibilityof a return current to the resonant circuit. A limiter allows tomaintain the power supply voltage Vdd below a threshold, e.g. of 4V. Aresistor is advantageously placed between Vdb and the diode so as toisolate the modulated signal on Vdb. Thus, the demodulation of themodulated signal coming from the reader is performed directly from thesignal Vdb at the output of contactless device's rectifier bridge.

FIG. 2 is a schematic illustration of a classical amplitude demodulationdevice.

The signal Vdb is first of all processed by an RC type low-pass filterso as to eliminate the components of the carrier and extract therefromon the one hand the envelope of the modulated signal, generally referredto as the reference modulating signal Vmod, and on the other itscontinuous component DC. A cutoff frequency of a few tens of kHz can bechosen for that first filter. The continuous level DC is then extractedby another low-pass filter whose cutoff frequency is less than thefrequency of the modulating signal Vmod, e.g. a few kHz. Thedemodulation signal Vdemod can then be obtained by comparison betweenthe modulating reference signal Vmod and its continuous level DC.

Such a demodulation device presents a number of limitations due to itsvery structure. Indeed, the continuous level DC varies greatly as afunction of the position of the card in the field of the-reader, and ofits displacement speed. This situation makes it difficult to generate areliable comparison level on a permanent basis. This problem is evenmore acute in applications where a large range, about 50 cm to 1 m, isrequired.

The graphs of FIGS. 3 a to 3 c illustrate the limits of classicaldemodulation devices.

The reference modulating signal Vmod is represented on FIG. 3 a with itscontinuous level DC. The demodulation signal Vdemod is shown in FIG. 3b. It is observed that certain modulations can fail to be detected andthat the modulations identified by the device are not perfectlyidentified, i.e. the start and end of modulation are not identified withprecision.

FIG. 3 c illustrates to this end the aim of the present invention, whichconsists in producing, for a contactless device, a demodulator capableof identifying with precision the start and end of all the modulatedsignals sent by the reader.

SUMMARY OF THE INVENTION

An object of the present invention is to overcome the drawbacks of theprior art, and to propose an active demodulator based on the “peak andtrough detection ” principle.

The demodulator according to the invention is termed “active”, i.e. itis capable of generating, by following the dynamically modulated signal,a low (lower) threshold and a high (higher) threshold which allow toidentify precisely the start and end of demodulation.

Such a demodulator is principally comprised of two independentdemodulators that are respectively optimized to detect the start and endof the modulated signal.

The invention more particularly has for an object a demodulator for anamplitude modulated signal, characterised in that it comprises:

-   -   a peak detection cell for extracting the reference modulating        signal from the modulated signal;    -   a first demodulator for detecting the peak of the reference        modulating signal to generate an upper comparison threshold and        identify the start of modulation;    -   a second demodulator for detecting a trough of the reference        modulating signal to generate a lower comparison threshold and        identify the end of modulation; and    -   a logic processing unit for supplying the demodulated signal.

According to a characteristic, the peak detection cell comprises:

-   -   a diode to which is input the modulated signal, and    -   a capacitor in parallel with a current source,    -   the reference modulating signal representing the charging and        discharging voltage at the terminals of the capacitor.

According to a characteristic, the peak detection cell further producesa shifted copy of the reference modulating voltage.

According to a characteristic, the first demodulator produces a shiftedcopy of the reference modulating signal and comprises a detection cellfor detecting the peak of the shifted reference signal so as to generatean upper comparison threshold, a comparator being provided to detect thecrossing between the upper threshold and the shifted reference signal todefine the start of modulation.

According to a preferred embodiment, the peak detection cell of thefirst demodulator comprises at least one transistor which is conductingin the absence modulation and blocking during modulation, the transistorbeing connected in series with a capacitor in parallel with a currentmirror, the start of modulation being identified by the discharge of thecapacitor by the current mirror.

According to a specific aspect, the upper comparison threshold isgenerated by a shift of the discharge voltage at the terminals of acapacitor.

According to an embodiment, the slope of the discharge is comprisedbetween 50 and 100 kV/s.

According to another characteristic, the second demodulator produces ashifted copy of the reference modulating signal and comprises a cell fordetecting troughs of the shifted reference signal so as to generate thelower comparison threshold, a comparator being provided to detect thecrossing between the lower threshold and the shifted reference signal todefine the end of modulation.

According to a preferred embodiment, the trough detection cell of thesecond demodulator comprises at least one transistor which is blockingin the absence of modulation and conducting during modulation, thetransistor being connected in parallel with a capacitor and in serieswith a current mirror, the end of modulation being identified by therecharge of the capacitor by the current mirror.

According to a specific aspect, the lower comparison threshold isgenerated by a shift of the charging voltage at the terminals of acapacitor.

According to an embodiment, the charging slope is comprised between 50and 100 kV/s.

According to another characteristic, the logic processing unit comprisestwo latches controlled respectively by the comparators of the peakdetection demodulator and the trough detection demodulator.

According to another characteristic, the demodulator further comprises afeedback control system for maintaining the reference modulating signalbelow the supply voltage of the demodulator.

The present invention also has for an object a contactless device, ofthe IC card type, comprising such a demodulator.

According to a characteristic, the device is capable of communicatingwith a read/write device over a distance that can reach 1 m with adisplacement speed that can reach 3 m/s.

The demodulator according to the invention allows to frame veryprecisely the start and end of modulation. It is thus possible to befree from the constraints linked to large variations in dc levels of themodulated signals.

Accordingly, the demodulator according to the invention can be used inapplications for which a large range and high displacement speed arerequired between the contactless device and the reader.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the present invention shallbecome apparent from the description that follows, given as anillustrative and non-limiting example with reference to the drawings inwhich:

FIG. 1, already described, is a block diagram of the input stages of thecontactless device;

FIG. 2, already described, is a classical functional diagram for thedemodulation of a signal;

FIGS. 3 a to 3 c, already described, are graphs illustratingrespectively the modulated signal and its dc level, the signaldemodulated with a classical device and the demodulated signal with ademodulator according to the invention;

FIG. 4 is a functional diagram of the demodulator according to theinvention;

FIG. 5 is a functional diagram of a basic cell for the “peak ”detection;

FIG. 6 is a graph illustrating the reference modulating signal of themodulated signal;

FIG. 7 is a block diagram of the implementation of the demodulatoraccording to the invention in a contactless device;

FIG. 8 is a schematic illustration of the detection of the start ofmodulation by the demodulator according to the invention;

FIG. 9 is a schematic illustration of the detection of the end ofmodulation by the demodulator according to the invention;

FIG. 10 illustrates graphically the signal demodulated by thedemodulator according to the invention;

FIG. 11 is a diagram of the logic unit of the demodulator according tothe invention;

FIGS. 12 a to 12 c are graphs illustrating respectively the modulatedsignal, the reference modulating signal and the signal demodulated bythe demodulator according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The description that follows makes reference to a contactless cardreceiving from the reader an amplitude modulated signal whose carrierfrequency is 13.56 MHz and whose modulation index is 10%. Theapplication requires an RF communication range capable of attaining 1 mfor a card displacement speed in the field of the reader that can attain3 m/s.

Referring to FIG. 4, which is a block diagram of the demodulatoraccording to the invention, the modulated signal Vdb is first of allprocessed by a “peak detection ” cell DCR, which extracts the referencemodulating signal Vpeak.

Two independent demodulators FE and RE arranged in parallel then detectrespectively the start and end of the modulation, these intermediateresults are then digitally processed by logic to produce the demodulatedsignal.

The first demodulator shall hereafter be designated “Falling Edge” FE,since it detects the upper threshold of the reference modulating signalVpeak and identifies the start of the modulation upon a drop in voltagein Vpeak (drop produced by the 10% modulation in the exampleconsidered).

The second demodulator shall hereafter be designated “Rising Edge” RE,since is detects a trough in the reference modulating signal Vpeak(trough corresponding to the 10% modulation) to generate a lowerthreshold specific to that modulation and identify the end of themodulation upon the rising again of the voltage Vpeak.

There is thus obtained a dynamic demodulator that follows the evolutionof the modulated signal to repeat in real time the start and end of themodulation.

The demodulator according to the invention comprises a number ofelectronic units for its implementation. These different blocks areillustrated by circuit diagrams in FIG. 7. This figure represents justone example of an implementation in a contactless card or electronicbadge and must in no case be seen as restrictive.

The first implementation step consists in extracting a referencemodulating signal (an envelope) from the modulated signal Vdb so as toeliminate the carrier from the latter.

To this end, there is performed a first peak detection DCR from a basiccell shown schematically in FIG. 5. Such a cell is essentially composedof a diode, a capacitor and a direct current source, and enables toextract the reference modulating signal Vpeak from the modulated signalVdb.

When dealing with a double alternating modulated signal Vdb, thecapacitor C can only be charged during a positive cycle of Vdb, i.e.when Vdb−Vpeak>Vd, where Vd is the diode's threshold voltage. As soon asthis relation no longer holds, the diode of the basic cell cuts off andthe capacitor C has a memory effect, i.e. at the moment when the diodecuts off, there is very precisely Vpeak=vdb(peak)−Vd. Capacitor C isthen discharged at constant current and Vpeak decreases according to therelation dVpeak/dt=−i/C.

It is consequently necessary to choose an appropriate time constant, ofa few tens of kHz, for. example, to recover the modulated signal withoutattenuation, while limiting the resultant due to the carrier. Indeed,the frequency of the modulated signal is slow in comparison to that ofthe carrier.

FIG. 6 is a graph illustrating the reference modulating signal Vpeakextracted from the modulated signal Vdb by the basic cell of FIG. 5.

If V1 is the modulation depth (10% for example) and V2 is the resultantof the carrier on the extracted signal Vpeak, V2 can be considered asthe noise level on Vpeak. With a carrier frequency Fc, it is possible toestimate the value V2(max)=i/(C*Fc), which corresponds to the dischargeof the capacitor over a complete period of the carrier.

In the implementation according to the example of FIG. 7, the referencemodulating signal Vpeak1 is directly extracted from the modulated signalVdb coming from the antenna. The parameters of the current mirror M4 andof the capacitor C13 are defined to obtain a discharge slope of Vpeak1of 600 kV/a.

Further to different simulations produced with several contactlesscards, it was possible to establish a maximum modulation of 500 mV in 1μs, i.e. a slope of 500 kV/s, which is less than the discharge slope(600 kV/s) of the peak detection cell DCR of the demodulator inaccordance with the invention.

The resultant of the carrier V2 can be estimated at 44 mV for a carrierfrequency of 13.56 MHz. The noise level is thus acceptable for amodulation depth of 500 mV.

An image Vpeak2 of the reference modulating signal Vpeak1 is produced bymeans of a simple shift using a diode. It is these two signals Vpeak1and Vpeak2 that shall be used subsequently respectively by demodulatorsFE and RE, whose results shall be processed by a logic unit which is notshown in FIG. 7.

Note that the demodulator according to the invention is furthermoreprovided with a feedback control system which allows to set thereference modulating signal Vpeak1 to the highest possible point ofdynamic admissible for the demodulator, irrespective of its powersupply, which can be comprised between 2V and 5V, for example. A highvoltage transistor M9. draws through a resistor RX3 a current which isvariable as a function of the modulated signal level Vdb. There is thusobtained a variable shift between Vdb and the power supply.

Reference shall now be made to FIGS. 7 and 8, which illustrate theoperation of the demodulator FE that identifies the start of modulation.

This demodulator FE is also based on the peak detection principle, butthe diode is replaced by an NMOS transistor in a follower configuration.As illustrated in FIG. 7, the transistors M17 and M18 are transistorsconnected as followers and biased by current mirrors M5 and M6.

Transistor M17 produces a copy of the reference modulating signal Vpeak1at Vmodul with a shift of Vt corresponding to the threshold voltage oftransistor M17, e.g. of 0.7V.

Transistor M18 acts like the diode of the peak detection basic cell DCRto create an upper threshold Vdcmod on signal Vmodul. Vdcmod representsthe voltage across the terminals of capacitor C14.

In the absence of modulation, M18 charges capacitor C14 and there isthen Vdcmod=Vmodul (Vdcmod is on the peak of the modulated signal). Whenthe amplitude modulation starts, the voltage Vpeak1 drops and there isthen Vpeak1−Vt<Vmodul. Transistor M28 switches off and capacitor C14 isdischarged at constant current by the current mirror M5 which biasestransistor M18. Vdcmod then represents the discharge of capacitor C14 atconstant current. The discharge slope of Vdcmod is fixed at 75 kV/s.

This discharge is slow relative to the modulation, but should however befaster than a possible drop in the power supply when the card moves awayfrom the reader. Considering a card having a range of 50 cm moving at 3m/s relative to the reader, and considering to a first approximationthat the internal supply voltage is proportional to the communicationdistance between the label and the reader, there can be a variation of2V to 4V over 50 cm in ⅙ s, which gives a minimum discharge slope of 12V/s, to compare with 75 kV/s.

Vdcmod identifies the peak of signal Vmodul. To create a comparisonthreshold, Vdcmod is reproduced at Vdcmodshift using a potential dropacross a resistor RX0 through which passes the biasing current. Therecan thus be obtained a shift of around 110 mV before hysteresis betweenVdcmod and Vdcmodshift.

The start of modulation is determined by the crossing of Vmodul withVdcmodshift. These two signals are processed by a comparator exhibitinga hysteresis of 50 mV, sufficient to eliminate possible multipletransitions due to the resultant of the carrier. The threshold fortriggering the detection of the start of modulation is thus of around160 mV, which is relatively precise if there is a modulation depth of500 mV, and cannot be confused with a resultant of the carrier estimatedat 44 mV.

Likewise, the detection of end of modulation by demodulator RE isillustrated in its principle in FIG. 9.

While demodulator FE tracks the peak of the modulated signal in order togenerate a threshold lower than the latter and to identify the start ofmodulation, demodulator RE tracks the troughs of the modulated signal(during which there is modulation) in order to generate a thresholdhigher than the latter and identify the end of modulation.

To this effect, it is necessary to be able to discharge a capacitorquickly in order to memorise the level of the trough, and then torecharge that capacitor at constant current to create a recharging slopethat can generate a comparison threshold.

This is achieved by NMOS transistor M31 and PMOS transistors M26 and M27in a follower configuration and biased by current mirrors M30, M25 andM28. Transistor M31 copies Vpeak2 at Vmodulshift, and transistor M27copies Vmodulshift at Vmodinf with a shift Vtp equal to approximately1V.

The object of this double copying is to re-center signal Vmodinf, whichshall be used for the comparisons, within the dynamic of the system.Indeed, the feedback control maintains Vpeak1 approximately at the levelof the power supply. Vmodulshift is the shifted copy of Vpeak2, i.e. theimage of Vpeak1 shifted by a diode threshold plus an NMOS transistorthreshold, that is a shift of 1.4V. Now, given that the detection indemodulator RE is carried out by PMOS transistors, it is necessary toraise the levels of the signals, which is why one works with Vmodinfwhich corresponds approximately to Vpeak1−0.4V, and which then remainsin the input dynamic of the comparator of demodulator RE.

The lower threshold at the trough of signal Vmodinf shall be representedby the voltage at the terminals of capacitor C19, Vdcmodinf.

In the absence of modulation, capacitor C19 is charged. When modulationoccurs, Vmodulshift drops and becomes lower than Vdcmodinf−Vtp.Transistor M26 becomes conducting and rapidly discharges C19 so long asVdcmodinf does not return to the level of Vmodulshift+Vtp, that isVdcmodinf=Vmodinf. There is thus indeed Vdcmodinf in the trough of theimage of the modulated signal Vmodinf.

When the modulation ceases, Vmodulshift rises again and switches offtransistor M26, which no longer draws current on capacitor C19. Thelatter can then be recharged at constant current by current mirror M25.The time constant of demodulator RE is chosen to be identical to that ofdemodulator FE, that is 75 kV/s in the example considered.

Signal Vdcmodinf identifies the trough of the signal Vmodinf. As before,to create a comparison threshold, Vdcmodinf is copied on Vdcmodinfshiftby a drop in potential across a resistor RX4 through which passes thebiasing current. There can thus be obtained a shift of approximately 75mV before hysteresis between Vdcmodinf and Vdcmodinfshift.

End of modulation is determined by the crossing of Vmodinf withVdcmodinfshift. These two signals are processed by the same comparatoras the one used for demodulator FE, exhibiting a hysteresis of 50 mV.The triggering threshold for the detection of the end of modulation isthus approximately 125 mV.

The signal demodulated by the demodulator according to the inventioncorresponds to the logic processing of the outputs of the twocomparators of the two demodulators FE and RE, as shown in FIG. 11.

FIG. 10 illustrates graphically the output signals of the comparators.An image of the modulated signal is given by the graph “modul”, theoutput of the comparator of demodulator FE is represented by the graph“demodFE” which identifies precisely the start of modulation, and theoutput of the comparator of demodulator RE is represented by the graph“demodRE” which identifies precisely the end of modulation. Thedemodulated signal is represented at the last graph.

The logic of the demodulator in accordance with the invention isdesigned to detect the first crossing between the modulated signal andthe comparison thresholds, the second crossing representing no usefulinformation in the application considered.

As illustrated in FIG. 11, the first crossing detected by comparator FEactivates a first latch D1 which thus announces the start of modulation,and the first crossing detected by comparator RE activates a secondlatch. D2 which thus announces the end of modulation.

FIGS. 12 a to 12 c are graphs illustrating simulations of thedemodulation of a signal Vdb with a demodulator in accordance with theinvention. Graph 12 c illustrates well the fine degree of precision ofthe demodulation.

1. A demodulator for an amplitude modulated signal comprising: a peakdetection cell (DCR) for extracting a reference modulating signal fromthe amplitude modulated signal; a first demodulator for detecting a peakof the reference modulating signal to generate an upper comparisonthreshold and identify a start of modulation; a second demodulator fordetecting a trough of the reference modulating signal to generate alower comparison threshold and identify an end of modulation; and alogic processing unit connected to said first and second demodulatorsfor supplying a demodulated signal.
 2. A demodulator according to claim1 wherein said peak detection cell (DCR) comprises: a diode receivingthe amplitude modulated signal; a current source connected to saiddiode; and a capacitor having terminals in parallel with said currentsource so that the reference modulating signal represents a charging anddischarging voltage at the terminals of said capacitor.
 3. A demodulatoraccording to claim 1 wherein said peak detection cell (DCR) furtherproduces a shifted copy of the reference modulating signal.
 4. Ademodulator according to claim 1 wherein said first demodulator producesa first shifted copy of the reference modulating signal and comprises: apeak detection cell for detecting peaks of the first shifted copy of thereference modulating signal to generate the upper comparison threshold;and a first comparator to detect a crossing between the upper comparisonthreshold and the first shifted copy of the reference modulating signalto define the start of modulation.
 5. A demodulator according to claim 4wherein said peak detection cell comprises: a first capacitor; a firstcurrent mirror connected in parallel with said first capacitor; and atleast one first transistor connected in series with said first capacitorand conducting in an absence modulation and blocking during modulationso that the start of modulation is identified by a discharge of saidfirst capacitor by said first current mirror.
 6. A demodulator accordingto claim 5 wherein the upper comparison threshold is generated by ashift of the discharge of said first capacitor.
 7. A demodulatoraccording to claim 6 wherein the discharge has a slope between 50 and100 kV/s.
 8. A demodulator according to claim 1 wherein said seconddemodulator produces a second shifted copy of the reference modulatingsignal and comprises: a trough detecting cell for detecting troughs ofthe second shifted copy of the reference modulating signal to generatethe lower comparison threshold; and a second comparator to detect acrossing between the lower comparison threshold and the second shiftedcopy of the reference modulating signal to define the end of modulation.9. A demodulator according to claim 8 wherein said trough detection cellcomprises: a second capacitor; a second current mirror connected inseries with said second capacitor; and at least one second transistorconnected in parallel with said second capacitor and blocking in anabsence modulation and conducting during modulation so that the end ofmodulation is identified by a recharge of said second capacitor by saidsecond current mirror.
 10. A demodulator according to claim 9 whereinthe lower comparison threshold is generated by a shift of the rechargeof said second capacitor.
 11. A demodulator according to claim 10wherein the recharge has a slope between 50 and 100 kV/s.
 12. Ademodulator according to claim 1 wherein said first demodulatorcomprises a peak detection cell and a first comparator connectedthereto; wherein said second demodulator comprises a trough detectioncell and a second comparator connected thereto; and wherein said logicprocessing unit comprises first and second latches controlledrespectively by said first and second comparators.
 13. A demodulatoraccording to claim 1 further comprising a feedback control system formaintaining the reference modulating signal below a supply voltage. 14.A demodulator for an amplitude modulated signal comprising: a peakdetection cell (DCR) for extracting a reference modulating signal fromthe amplitude modulated signal; a first demodulator comprising a peakdetection cell for detecting peaks of a first shifted copy of thereference modulating signal to generate an upper comparison threshold,and a first comparator to detect a crossing between the upper comparisonthreshold and the first shifted copy of the reference modulating signalto define a start of modulation; a second demodulator comprising atrough detecting cell for detecting troughs of a second shifted copy ofthe reference modulating signal to generate a lower comparisonthreshold, and a second comparator to detect a crossing between thelower comparison threshold and the second shifted copy of the referencemodulating signal to define an end of modulation; and a logic processingunit connected to said first and second demodulators for supplying ademodulated signal.
 15. A demodulator according to claim 14 wherein saidpeak detection cell (DCR) comprises: a diode receiving the amplitudemodulated signal; a current source connected to said diode; and acapacitor having terminals in parallel with said current source so thatthe reference modulating signal represents a charging and dischargingvoltage at the terminals of said capacitor.
 16. A demodulator accordingto claim 14 wherein said peak detection cell comprises: a firstcapacitor; a first current mirror connected in parallel with said firstcapacitor; and at least one first transistor connected in series withsaid first capacitor and conducting in an absence modulation andblocking during modulation so that the start of modulation is identifiedby a discharge of said first capacitor by said first current mirror. 17.A demodulator according to claim 14 wherein said trough detection cellcomprises: a second capacitor; a second current mirror connected inseries with said second capacitor; and at least one second transistorconnected in parallel with said second capacitor and blocking in anabsence modulation and conducting during modulation so that the end ofmodulation is identified by a recharge of said second capacitor by saidsecond current mirror.
 18. A demodulator according to claim 14 whereinsaid logic processing unit comprises first and second latches controlledrespectively by said first and second comparators.
 19. A demodulatoraccording to claim 14 further comprising a feedback control system formaintaining the reference modulating signal below a supply voltage. 20.A contactless device comprising: a peak detection cell (DCR) forextracting a reference modulating signal from an amplitude modulatedsignal; a first demodulator for detecting a peak of the referencemodulating signal to generate an upper comparison threshold and identifya start of modulation; a second demodulator for detecting a trough ofthe reference modulating signal to generate a lower comparison thresholdand identify an end of modulation; and a logic processing unit connectedto said first and second demodulators for supplying a demodulatedsignal.
 21. A contactless device according to claim 20 wherein said peakdetection cell (DCR) comprises: a diode receiving the amplitudemodulated signal; a current source connected to said diode; and acapacitor having terminals in parallel with said current source so thatthe reference modulating signal represents a charging and dischargingvoltage at the terminals of said capacitor.
 22. A contactless deviceaccording to claim 20 wherein said peak detection cell (DCR) furtherproduces a shifted copy of the reference modulating signal.
 23. Acontactless device according to claim 20 wherein said first demodulatorproduces a first shifted copy of the reference modulating signal andcomprises: a peak detection cell for detecting peaks of the firstshifted copy of the reference modulating signal to generate the uppercomparison threshold; and a first comparator to detect a crossingbetween the upper comparison threshold and the first shifted copy of thereference modulating signal to define the start of modulation.
 24. Acontactless device according to claim 23 wherein said peak detectioncell comprises: a first capacitor; a first current mirror connected inparallel with said first capacitor; and at least one first transistorconnected in series with said first capacitor and conducting in anabsence modulation and blocking during modulation so that the start ofmodulation is identified by a discharge of said first capacitor by saidfirst current mirror.
 25. A contactless device according to claim 20wherein said second demodulator produces a second shifted copy of thereference modulating signal and comprises: a trough detecting cell fordetecting troughs of the second shifted copy of the reference modulatingsignal to generate the lower comparison threshold; and a secondcomparator to detect a crossing between the lower comparison thresholdand the second shifted copy of the reference modulating signal to definethe end of modulation.
 26. A contactless device according to claim 25wherein said trough detection cell comprises: a second capacitor; asecond current mirror connected in series with said second capacitor;and at least one second transistor connected in parallel with saidsecond capacitor and blocking in an absence modulation and conductingduring modulation so that the end of modulation is identified by arecharge of said second capacitor by said second current mirror.
 27. Acontactless device according to claim 20 wherein said first demodulatorcomprises a peak detection cell and a first comparator connectedthereto; wherein said second demodulator comprises a trough detectioncell and a second comparator connected thereto; and wherein said logicprocessing unit comprises first and second latches controlledrespectively by said first and second comparators.
 28. A contactlessdevice according to claim 20 further comprising a feedback controlsystem for maintaining the reference modulating signal below a supplyvoltage.
 29. A contactless device according to claim 20 capable ofcommunicating with a read/write device over a distance of up to 1 m andwith a displacement speed of up to 3 m/s.
 30. A demodulation methodcomprising: extracting a reference modulating signal from an amplitudemodulated signal; detecting a peak of the reference modulating signal togenerate an upper comparison threshold and identify a start ofmodulation; detecting a trough of the reference modulating signal togenerate a lower comparison threshold and identify an end of modulation;and performing logic processing for supplying a demodulated signal basedupon the extracting and detecting.
 31. A method according to claim 30wherein detecting a peak comprises: generating a first shifted copy ofthe reference modulating signal; detecting peaks of a first shifted copyof the reference modulating signal to generate the upper comparisonthreshold; and detecting a crossing between the upper comparisonthreshold and the first shifted copy of the reference modulating signalto define the start of modulation.
 32. A method according to claim 30wherein detecting a trough comprises: generating a second shifted copyof the reference modulating signal; detecting troughs of the secondshifted copy of the reference modulating signal to generate the lowercomparison threshold; and detecting a crossing between the lowercomparison threshold and the second shifted copy of the referencemodulating signal to define the end of modulation.
 33. A methodaccording to claim 30 further comprising maintaining the referencemodulating signal below a supply voltage.